source: How long is a day?⌚ | How long is a day?⌚ | By StarTalk | Facebook
Why
- We can’t define time based on the rotation of the Earth, because it’s not stable.
- If Earth slow down or speed up, we could never not know.
Hence
- We use the vibration of Cesium 137: there is a electron transition between 2 energy levels that has a very precise frequency.
- And when we measure, it turns out that The Earth is slowing down.